The present invention relates to heat exchangers and more particularly relates to a temperature control system for maintaining a constant temperature in a heat detector of a heat exchanger.
In a rotary regenerative heat exchanger, a mass of heat absorbent material commonly comprised of packed element plates is positioned in a hot exhaust gas passageway to absorb heat from the hot gases passing therethrough. After the plates become heated by the gas they are positioned in a passageway being traversed by cool air where heat is transferred from the heated plates to the cool air or gas flowing therethrough.
The heat-containing gases are typically the exhaust gases from a combustion process. As the hot exhaust gases are directed through the rotary regenerative heat exchanger, fly ash and unburned products of combustion carried by the exhaust gas are deposited on the surface of the packed element plates. The deposits continue to build up until the rate of air and gas flow through the heat exchanger is reduced in at least the region of the build-up. When the temperature is elevated to the ignition point of the deposit, heat is then generated until the deposits begin to glow and cause a "hot spot", that if not detected will rapidly increase in temperature until the metal of the heat exchanger will itself ignite and cause a fire. U.S. Pat. Nos.: 3,730,259; 3,861,458; 4,022,270; 4,383,572 and 4,813,003; the disclosure of each being hereby incorporated by reference, disclose apparatus to detect hot spots in the packed element plates of a rotary regenerative heat exchanger.
Hot spot detectors frequently employ computerized infrared detectors to detect temperature changes within the exchanger. The infrared detectors frequently employ a lead sulfide chip which is itself sensitive to temperature changes. In order to maintain a consistent level of chip sensitivity, a temperature control system is employed to keep the detector at a constant temperature. The detector electronics are then calibrated for that particular temperature of the chip. In the past, the control system for maintaining a constant chip temperature has consisted of cooling water circulated through a jacket in the sensor head assembly. This type of system has been problematic, however, due to water leaks that ruin the detector, a lack of reliability in the water supply, and a variable water temperature. All of these factors lead to a lack of consistency in the temperature of the detector, which can lead to a lack of consistency in the detection of hot spots. Furthermore, while the system can be used to cool the detector, it is not capable of heating the detector.